Puncture potatoes in several places with a fork. Bake 1 hour in a 180ºC oven or until tender. Remove and discard skins. Mash potatoes through a ricer or food mill into a large bowl, and let cool a little. Add egg yolk, salt, and 2 cups flour. Mix well. Knead dough on work surface or board into a ball. It should be soft and a little sticky. If it's too sticky, add a little flour. Lightly flour surface and hands. Break dough into egg-size pieces. Roll until thickness of thumb. Cut roll into 3 cm pieces. Push each Gnocchi against the back of a fork to imprint a pattern. This helps them absorb more sauce (and makes them a little prettier). Arrange them on a floured tray. Fill a large saucepan two third full with salted water. Bring to a boil. Add oil and dumplings. Stir, to make sure they don't stick t bottom. When they come to surface, let them cook 10-12 seconds more. Remove with a slotted spoon as they are done. They get soggy if overcooked (they absorb more moisture than regular pasta). Serve with butter and Parmesan cheese or your favorite sauce.